1. Field of the Invention
The present invention concerns a device for optical inspection of moving material. It applies to all materials which bear a lattice and in particular textiles, cloth, wallpaper, plastic, printed materials or dyed weaves.
2. Related Art
Inspection devices known at present have an electronic camera or an array of photosensitive components, generally linear, the optical field of which is constant, in width and time frequency of view capture.
The duration between the start of two successive view captures is therefore independent of possible changes in speed of running of the material or changes in pitch of the lattice borne by successively inspected materials. The two lattices, that of the individual inspection areas, linked to the optoelectronic system, and that of the textile, interfere and prevent any fine inspection.
This is because, on the one hand, when the textile lattice has a mesh which is smaller than that of the optoelectronic system, a defect on only one element of the textile lattice cannot be detected by the inspection system since it causes too small a variation in signal. On the other hand, when the textile lattice has a mesh which is larger than that of the optoelectronic system, the latter detects each space between the elements of the textile lattice and interprets it as a perforation in the lattice.
French patent application FR 93 02279 is known which describes a method and device for woven textile fabric image processing. Each fabric having a repetitive design and being defined by its period, the method of this document consists of acquiring an image and filtering it by subtracting, at each digital illumination value of a point of the image corresponding to a point of the fabric, the digital illumination value of another point of the image which is substantially displaced by one period of the cloth structure. This is intended to avoid the effects due to the cloth framework, that is to say the design which is periodically reproduced on the cloth, during weaving.
The filtering thus performed is purely electronic and is performed a posteriori on an image which, at view capture, has no particular characteristic apart from being in synchronised with the running of the cloth so that each view capture corresponds to the same distance travelled by the cloth.
Since the image capture performed by this camera is synchronised with the speed of running of the fabric, between two view captures, the same length of fabric is passed under the camera, whatever the speed of movement of the fabric. However, between two points of one image or between two points of two images, the number of threads or the number of lattice pitches or the number of framework periods which have passed in the camera field between two view captures is not an integer number or the inverse of an integer number, but only a constant number, unvarying with the speed of the cloth.
Thus the lattices of the points observed during view captures and that of the cloth threads have the same defect as described above: two points at which the illumination values are subtracted, since they are approximately one period apart, may correspond one to an interstice between threads and the other to the centre of a thread. The result of the difference then gives a high value even though the cloth has no weaving defect. Conversely, a defect, such as a thick thread which fills up the said interstice, may correspond to a small difference in illuminations. It is then not detected according to the method of the document. Again, the smallest defects may not be detected by this method.
Consequently, no solution is satisfactory and only a few extremely visible defects are detected by these systems, that is to say holes in the textiles and possibly strongly contrasting threads.
The present invention intends to remedy these drawbacks by synchronising the duration between two view captures of the image sensor and the duration of passage of one or more pitches of the lattice borne by the material.